Hot melt adhesives (HMAs) are a form of thermoplastic adhesive that are designed to be applied in the molten state. The glue is tacky when hot, and solidifies from a few seconds to several minutes. Hot melt adhesives may be applied by such methods as slot-coating, roll-coating, extrusion, dipping or spraying.
In industrial use, hot melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. Hot melt adhesives have long shelf life and usually may be disposed of without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates sensitive to higher temperatures, and loss of bond strength at higher temperatures, due to significant, and up to complete melting of the adhesive. Some HMAs may not be resistant to chemical attacks and weathering. HMAs do not lose thickness during solidifying; solvent-based adhesives may lose from up to 50%-70% of layer thickness during drying.
Hot melt adhesives formulations of the prior art include styrene block copolymer (SBC, aka rubber)-based formulations, acrylic-based (acrylics) formulations, silicone-based formulations, and metallocene polyethylene- and metallocene polypropylene-based formulations.
Amorphous poly alpha olefin (APAO) polymers are compatible with many plasticizers, tackifiers, waxes, and polymers; they find wide use in many adhesive applications. APAO hot melt adhesives have good acid resistance, moderate heat resistance, light and UV resistance due to their saturated hydrocarbon nature, are tacky, soft and flexible, have good adhesion to multiple substrates and longer open times than crystalline polyolefins. APAOs tend to have lower melt viscosity, better adhesion, longer open times and slower set times than comparable ethylene vinyl acetate (EVA) copolymers. Some APAOs may be used alone, but often they are compounded with co-adjuvants such as tackifiers, waxes, and plasticizers (for example, mineral oil, polybutylene oil and naphthenic oil).
Amorphous poly alpha olefins are produced by the (co-)polymerization of α-olefins, for example, ethylene (CAS# 74-85-1), propylene (CAS# 115-07-1), butene-1 (CAS# 106-98-9) or hexene-1 (CAS# 592-41-6), with Ziegler-Matta catalysts. The (co)-polymers have an amorphous structure which makes them useful for the production of hot melt adhesives.
Examples of amorphous poly alpha olefins include for example, amorphous (also known as atactic) polypropylene (APP, CAS# 9003-07-0), amorphous poly propylene-co-ethylene (APE, CAS# 9010-79-1), amorphous poly propylene-co-butene-1 (APB, CAS# 29160-13-2), amorphous poly propylene-co-hexene-1 (APH, CAS# 25895-44-7) copolymers, and amorphous poly propylene-co-ethylene-co-butene-1 (APEB, CAS# 25895-47-0) and amorphous poly propylene-co-butene-1-co-hexene-1 (APBH) terpolymers. APP is harder than APE, which is generally harder than APB, which is generally harder than APH, in accordance with decreasing crystallinity. And in accordance to their decreasing crystallinity, APP has higher tensile or mechanical strength than APE, which has generally higher tensile or mechanical strength than APB, which has generally higher tensile or mechanical strength than APH. Due to their lower molecular weights than other polymers such as styrene block copolymers (SBCs), or acrylics or many metallocene polyethylenes and polypropylenes, at typical application temperatures of 375 degrees Fahrenheit, APAOs exhibit a high degree of substrate wetting which is a very desirable HMA property. However, APAOs show relatively low cohesion, the entangled polymer chains having a fairly high degree of freedom of movement. Under mechanical load, most of the strain is dissipated by elongation and disentanglement of polymer chains, and only a small fraction reaches the adhesive-substrate interface. Cohesive failure rather than adhesive failure is therefore a more common failure mode of APAOs.
The present embodiments meet these needs.
The present embodiments are detailed below with reference to the listed Figures.